U.S. Pat. No. 4,019,818 to D. H. R. Vilkomerson discloses a quadrature-dual-interferometer which simultaneously employs two phase-quadrature beams of coherent light which interfere with a signal beam of coherent light from a spot of a displaceable signal mirror insonified by an ultrasonic wave and which permits an output signal to be derived which is proportional to the sum of the squares of the ultrasonic frequency component of the interference between the signal beam and each respective one of the reference beams. This output signal is inherently substantially proportional to the intensity of the ultrasonic wave then insonifying the spot of the signal mirror and independent of random phase drift and environmental vibrations in the interferometer. Thus, in this quadrature-dual-interferometer, by squaring and summing the signals from the two channels, a stable signal, proportional to the square of the displacement of the measuring mirror, is obtained.
The squaring operation destroys the phase information about the signal and imposes circuit difficulties in that the squaring operation doubles the frequency and cuts the dynamic range attainable. If the signal is of a bandwidth greater than an octave, the squaring procedure introduces distortion in the form of new components of the signal appearing due to cross-products between spectral components of the signal. Therefore there is a need for an improved arrangement which avoids these problems.
A preliminary search of the prior art relating to the subject matter of the present invention reveals the following U.S. patents of interest:
Rambauske, et al., U.S. Pat. No. 3,202,052
Mezrich, et al., U.S. Pat. No. 3,969,528
Wang, U.S. Pat. No. 4,005,935
Redman, et al., U.S. Pat. No. 4,005,936
Vilkomerson, U.S. Pat. No. 4,019,818